Q&A System Oversizing - Short #291 · HVAC School - For Techs, By Techs summary

Key takeaways

Oversized HVAC systems can cause humidity and comfort issues, which can be mitigated by proper airflow, system matching, and dehumidification.
In new homes, replacing the outdoor condenser rather than the entire system is a viable option if components are compatible, helping to address oversizing.
Decoupling latent and sensible loads with dedicated dehumidification can improve humidity control, but the primary focus should be on correctly sized equipment.
Intentionally derating HVAC capacity through methods like reheat or smaller compressors is possible but involves trade-offs such as higher energy costs and system complexity.
Proper equipment sizing, managing airflow, and load matching are essential to prevent moisture problems and ensure occupant comfort.

Summary

Understanding the Challenges of Oversized HVAC Systems

This section discusses the issues that oversized HVAC systems can cause, particularly in regards to humidity control and occupant comfort. Bryan explains that the degree of the problem depends on how badly the system is oversized, its turn-down or staging capabilities, and its latent capacity setup. To optimize performance with an oversized system, reducing airflow to around 350 CFM per ton is recommended, along with proper dehumidification mode settings. The risks of low airflow include increased duct and cabinet sweating.

Options for Dealing with Oversized Equipment in New Homes

Bryan highlights that in new constructions, it's sometimes feasible to replace just the outdoor condenser while keeping the existing air handler, provided they match in terms of performance specs. He suggests checking AHRI data and the manufacturer’s expanded performance charts to verify compatibility. Properly matching components can help mitigate some issues caused by oversizing without complete system replacement.

Dehumidification Strategies and Decoupling Latent and Sensible Loads

The episode emphasizes decoupling humidity control from cooling to improve comfort. Installing a dedicated dehumidifier can help, especially when properly ducted into the supply. Bryan stresses that proper sizing and setup of the HVAC system itself remains crucial, with dehumidifiers filling gaps during shoulder seasons. The idea of fully decoupling latent and sensible loads is acknowledged as impractical but useful when feasible.

Intentionally Derating HVAC Equipment to Improve Humidity Control

Bryan discusses various methods to intentionally reduce cooling capacity, such as reheat, electric or water coils, or installing a smaller compressor – though these approaches can be invasive or impractical, especially in new homes. He notes that these methods can help improve humidity control but come with trade-offs like increased power costs and potential system imbalance.

Key Principles for Proper Sizing and Load Management

The core message is the importance of right-sizing equipment, managing airflow, and balancing heat transfer to prevent moisture problems. Bryan advocates for load matching and avoiding the common pitfalls of oversized equipment, which leads to higher costs and subpar performance. He emphasizes that strategic staging and design considerations are ultimately the best solutions for moisture and comfort issues.

Show notes

In this short Q&A podcast episode, Bryan answers a question from Evan, a licensed mold assessment consultant, about system oversizing: What can be done when an HVAC system is oversized to control humidity and keep occupants comfortable?

The extent of the problem will depend on how badly the equipment is oversized, whether it has any turn-down or staging, and how the latent capacity is set up. To make the equipment work as best as it can, reducing the airflow to about 350 CFM per ton can help, as well as properly setting up dehumidification modes. The downside to slower airflow is a likely increase in cabinet and duct sweating.

In brand-new houses, it may be possible to swap the condenser (though it may be unlikely with the recent refrigerant change) and then drop airflow at the air handler. You can use AHRI's data to look for a possible match and then check the manufacturer's expanded performance data to verify whether a new condenser is a match.

It's also possible to decouple the latent and sensible loads by installing a dehumidifier. However, completely decoupling the loads is not always practical, as a properly-sized HVAC system is the main source of dehumidification (and the dehumidifier fills in the gaps). Dehumidifiers need to be installed properly (with the supply ducted into the supply duct).

Intentionally derating the equipment's cooling capacity, such as via reheat, will turn it into a better dehumidifier, but you're still adding sensible BTUs and need to be aware of the consequences of that, including higher power bills. You can use electric reheat, remove insulation, or remove shades from windows. Using a smaller compressor until the system can be replaced and downsized would achieve a similar effect but comes with other negative consequences.

Right-sizing equipment, load-matching, and managing air movement and heat transfer are ultimately the keys to preventing moisture problems.

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Q&A System Oversizing - Short #291